A general description of a commonly known speaker apparatus is shown in FIG. 1. In this type of speaker apparatus, there is provided a cabinet 1 which has an interior area which is divided by a partitioning plate 2 into a front chamber, or compartment, 3 and a rear chamber, or compartment, 4. A speaker unit 11 (speaker driver) is mounted on the partitioning plate 2 for improving low-range playback characteristics. The speaker unit 11 has a sound radiating direction which is substantially directed towards the front chamber 3, that is towards the front side of cabinet 1. There is an internal duct 101 which is mounted, or pierced, thru the partition plate 2 so as to extend from the front chamber 3 to the rear chamber 4. Duct 101 allows the passage, or communication, of sound waves between the front and rear chamber 3 and 4. The interaction between the speaker unit 11, front and rear chambers 3 and 4, and duct 101 is that of a resonance circuit. The internal duct 101 is bent at a mid portion to adapt itself to the depth of the front chamber 3. Front chamber 3 is adapted to communicate, or allow passage of sound, to outside the cabinet 1, in a forward direction (substantially the same direction as the sound radiating direction of the speaker unit 11) by an external duct 6 provided in a front plate 8 of the cabinet 1. The speaker unit 11 has a magnetic circuit 12 and a diaphragm 17. The external duct 6 is mounted within, or thru, an opening 5 which is formed in the front plate 8 of the cabinet 1.
FIG. 2 shows such a vibrating system comprised by such speaker unit in which a resistor R.sub.1, an inductance L.sub.2, compliances C.sub.2, C.sub.3 and C.sub.4 are connected in series with a vibrating source to form a loop, an inductance L.sub.1 provided by the inner duct 101 is connected in parallel with the resistance R.sub.1, the inductance L.sub.2 and the compliance C.sub.2 have connected thereto an inductance L.sub.3 provided by the external duct 6 that is connected in parallel with the compliance C.sub.4.
With the above-described speaker apparatus, the air flows thru internal duct 101 at an elevated velocity between front chamber 3 and rear chamber 4. As a result of the high velocity passage of air thru the internal duct 101, a hissing sound is produced at or near the ends of the internal duct 101. This hissing sound is unwanted and acts generally to deteriorate the quality of the reproduced sound. As an example of specifically generated hissing sounds for a given cabinet, consider the following. If the outer dimensions of the cabinet are so set that its width, depth and height are 200 mm, 300 mm and 600 mm, respectively, the capacity of the front compartment 3 is 15000 cm.sup.3 (15 liters), the capacity of the rear compartment 4 is 10000 cm.sup.3 (10 liters), the diameter of a circular diaphragm 17 of the speaker unit 11 is 16 cm and the diameter and the overall length of the internal duct 101 are 60 mm and 260 mm, respectively, the hissing sound of 40 to 50 Hz is produced.
Further, with the speaker apparatus having such an internal duct 101, braking cannot be applied over the entire frequency range, such that, if a larger input signal is applied in succession to the speaker unit, the sound quality tends to be deteriorated. That is, suppression of the high-range standing waves and inhibition of distortion due to second and third harmonics cannot be achieved sufficiently.